Evolutionary Patterns of Intersexual Power

Simple Summary Social inequality is pervasive in primates, with male-biased power often assumed as the default condition. We tested this assumption with a broad dataset and explored factors that might make some types of intersexual power more likely to evolve, such as males being larger than females and skewed sex ratios. We hypothesized that intersexual power can arise due to sex differences in fighting ability as well as economic demand for mating opportunities. We found that even though societies with power biased towards males are most common in primates, some lemurs, monkeys, and apes exhibit societies without male-biased power. Male-biased power was associated with males being larger than females and more females in social groups (i.e., high supply/low demand for females), whereas non-male-biased power was associated with low supply/high demand for mating opportunities with females. The last common ancestor of primates did not necessarily exhibit male-biased power, but the last common ancestor of monkeys, apes, and humans probably did exhibit male-biased power. Non-male-biased power probably evolved independently multiple times in primates. However, multiple traits favor male-biased power and fewer paths lead to societies with female-biased power or intersexual equality.Abstract Dominance and leverage are both possible causes of social inequality. If sexual dimorphism influences patterns of intersexual dominance, we predicted that highly dimorphic species are constrained to exhibit male-biased power (MP), but species with low sexual dimorphism are free to demonstrate a broader range of intersexual power relationships. If market effects influence intersexual leverage, we predicted that females have more power when group composition is more male-biased and estrus is asynchronous. We analyzed data on intersexual power, sexual dimorphism, expected estrous overlap, and sex ratio for 79 extant primate species using phylogenetic logistic regression and ancestral state reconstructions. Although MP is more common, every major primate clade includes non-MP species. MP was associated with greater body mass and canine length dimorphism and with female-biased sex-ratios. Low estrous overlap was associated with non-MP. Although MP was reconstructed as likely ancestral for anthropoids, the last common ancestor of this clade probably did not exhibit high sexual dimorphism. The last common ancestor of catarrhines was probably highly dimorphic, potentially constraining intersexual power relationships. Non-MP probably evolved multiple times in primates and may be less common because multiple traits are linked to MP while fewer traits are associated with female-biased power or equality.

Automated summary

This study examines intersexual power dynamics in primates and finds that while male-biased power is most common, there are societies without such bias. Factors such as differing fighting ability and mating opportunities contribute to power imbalances. Male-biased power is associated with males being larger and more females in social groups, while non-male-biased power is associated with a higher demand for mating opportunities with females. Non-male-biased power has evolved independently multiple times in primates.